The invention relates generally to fish diversion apparatus and systems for facilitating movement of fish through dams and more particularly to means for diverting downstream-moving fish away from hydroelectric turbines.
A continuing problem in the exploitation of hydropower is the facilitation of up and downstream movement of fish, particularly anadromous species, through water impoundments. Use of fish ladders and other forms of artificial outlets has aided upstream migration of salmon and other fish.
However, such devices have not proven very useful as downstream migration routes for their offspring. In large bodies of impounded water, salmon and steelhead smolts cannot easily find the artificial outlet or ladder. When water levels are high, many smolts pass over the dam spillways. This is acceptable if the dam is low and water flow sufficient. However, water flow is frequently inadequate during the seasons of downstream migration of the smolts. And, if the dam is very high, the smolts are exposed to highly-nitrogenated water below the dam as well as the possibility of injury during their fall. Many smolts are thus lost. The only other alternative is for the smolts to pass downstream through the generator turbines. However, many more smolts are lost in the process.
To minimize losses of smolts in the turbines, the fish agencies frequently require maintenance of a minimum spillway flow regardless of water supply. However, doing so causes a substantial reduction in power generating capacity, especially during periods of low water flow.
Smolt mortality from the foregoing causes can be 10% or more for each dam in a river that may contain half a dozen or more dams. Such a high cumulative mortality rate makes it difficult to maintain or replenish already depleted populations of anadromous fish. Thus, it is important to find a way to facilitate safe downstream passage of fish through dams without unduly impairing the generating capacity of the dams.
One proposed solution to the foregoing problem calls for positioning a moving or traveling screen across the forebay leading to the penstocks containing the turbines, for example, as disclosed in U.S. Pat. Nos. 2,169,249 and 4,064,048. In top plan view, the moving screen is positioned at a 45.degree. angle to current flow into the forebay. However, such a screen is a large, bulky and expensive apparatus. Also, by virtue of the need for its moving parts to operate continuously to prevent plugging with debris, such a screen is subject to breakdown.
A second proposal involved positioning of a louver screen generally parallel to the flow of water in the forebay across the entrances to all of the penstocks except the one at the downstream end of the forebay. A similar system using a separate bypass is disclosed in U.S. Pat. No. 2,826,897. A wall on the opposite side of the forebay from the louver screen converges toward the downstream end penstock, which is provided with leaf gates to retain fish therein. The object of this system is to move most of the fish with the flow past the louver-screened penstocks to the end penstock provided with a 50 cubic foot per second (cps) bypass. Testing over a two year period disclosed that about 75% of the fish entering the forebay were guided by the louvers into the end penstock. Since this system was only experimental, the turbine was not removed from the end penstock. Although many fish escaped through the bypass, many more fish passed through the turbine and were injured. The large capacity of the bypass reduced generating capacity of the end unit by over 10%. For permanent use, the fish agencies proposed removing the turbine from the end penstock and using the entire 450 cps water supply as a bypass flow. However, doing so would cost greatly in lost generating capacity.
Another approach was to provide an artifical spillway into a sump. A hump-backed skimmer screen was positioned between the reservoir and the sump with its hump at the water surface. Water flowing over the screen carried the fish into the sump. The fish and a portion of the water was discharged into the river below the dam while the rest of the water was pumped back into the reservoir. This screen requires apparatus for raising and lowering it to adjust to variations in water levels. A debris screening system disclosed in British Patent No. 487,803 is likewise susceptible to changes in water level, and therefore would be unreliable as a fish bypass.
It has also been proposed to individually screen the entrances of the penstocks. However, this approach is unsatisfactory because the screens would quickly plug with water borne debris and, by resisting water flow, substantially reduce the efficiency of power generation.
Yet another system that has been tried calls for only screening the upper portion of the entrance of each penstock. Such a screen, when viewed in a side elevational view, is positioned along the downwardly convergent ceiling of the penstock at approximately a 45.degree. angle from horizontal. The screen's upper end contacts the ceiling of the penstock entrance just downstream of the gate well, which serves as a bypass. The lower end of the screen, upstream of the gate well, is spaced well above the floor of the entrance to the penstock. A fixed screen version of this system did not work very well. The approach velocity of water to the screen was about 4 to 6 feet per second. The gate well, having its outlet opening at the level of the surface of the reservoir, had a very low bypass flow. As a result, many of the fish that did not swim beneath the screen were impinged on it and trapped there by the high velocity of the water flow through the screen. The screen was tiltable for cleaning and for releasing trapped fish but many such fish were badly descaled and thus lost. A traveling screen version works better but costs much more to build, operate and maintain. It still diverts no more than about 70% of fish from the turbine and some fish injury still occurs.
Accordingly, there remains a need for a satisfactory means for fish to migrate downstream past dams without passing through the generator turbines.